The invention relates to an energy storage device comprising a DC voltage supply circuit as well as to a method for providing a DC voltage from an energy storage device, in particular a battery cycloconverter which supplies a DC voltage to a vehicle electric system.
It is becoming apparent that electronic systems, which combine the new energy storage technologies with an electric drive technology, will be increasingly used in the future for stationary applications, such as, e.g., wind turbines or solar plants, as well as in vehicles, such as hybrid or electric vehicles.
The feed of multi-phase current into an electric machine is usually performed by an inverter in the form of a pulse width modulated inverter. To this end, a DC voltage provided by a DC link can be converted into a multi-phase AC voltage, for example a three phase AC voltage. The DC link is thereby fed by a strand consisting of series-connected battery modules. In order to be able to meet the requirements for power and energy necessary for a respective application, a plurality of battery modules is frequently connected in series in a traction battery.
The series circuit of a plurality of batteries has the problem that the entire strand is rendered useless if a single battery module malfunctions. Such a breakdown of the energy supply strand can lead to a breakdown of the whole system. Furthermore, power reductions in a single battery module occurring temporarily or permanently can lead to power reductions in the entire energy supply system.
A battery system comprising an integrated inverter function is described in the American patent publication U.S. Pat. No. 5,642,275 A1. Systems of this type are known under the name of multilevel cascaded inverters or also battery direct inverters (BDI). Such systems comprise DC sources in a plurality of energy storage module strands which can be directly connected to an electric machine or an electric network. In so doing, single-phase or multi-phase supply voltages can be generated. The energy storage module strands comprise a plurality of series-connected energy storage modules, wherein each energy storage module has at least one battery cell and an associated controllable coupling unit, which enables, as a function of control signals, the at least one battery cell associated in each case therewith to be bypassed or said at least one battery cell associated in each case therewith to be connected into the respective energy storage module strand. In so doing, the coupling unit can be designed in such a manner that said unit additionally enables the at least one battery cell associated in each case therewith to be connected with inverse polarity into the respective energy module strand or enables the energy storage module strand to be interrupted. By suitably actuating the coupling units, e.g. with the help of pulse width modulation, suitable phase signals for controlling the phase output can also be provided so that a separate pulse-controlled inverter can be omitted. The pulse-controlled inverter required for controlling the phase output voltage is thus for all intents and purposes integrated into the BDI.
In comparison with conventional systems, BDIs usually have a higher degree of efficiency, a greater reliability and a significantly lower harmonic content of the output voltage thereof. The reliability is inter alia ensured by virtue of the fact that defective, failed or inefficient battery cells can be bypassed in the energy supply strands by means of a suitable actuation of the coupling units associated therewith. The phase output voltage of an energy storage module strand can be varied by a corresponding actuation of the coupling units and, in particular, adjusted in a stepped manner. The stepped adjustment of the output voltage thereby results from the voltage of a single energy storage module, wherein the maximum possible phase output voltage is determined by the sum of the voltages of all of the energy storage modules of an energy storage module strand.
The German patent publications DE 10 2010 027 857 A1 and DE 10 2010 027 861 A1 disclose, for example, battery direct inverters comprising a plurality of battery module strands which can be directly connected to an electric machine.
A constant DC voltage is not available at the output of BDIs because the energy storage cells are allocated to different energy storage modules and the coupling devices thereof have to be selectively actuated in order to generate a voltage level. As a result of this allocation of the energy storage cells, a BDI can basically not be used as a DC voltage source, for example, for supplying electricity to an electrical system of an electric vehicle. Accordingly, the charging of the energy storage cells is not readily possible via a conventional DC voltage source.
There is therefore a need for an energy storage device comprising a DC voltage supply circuit and a method for operating the same, with which a DC voltage consumer can be supplied with a DC voltage from the energy storage device or, respectively, a DC voltage charging circuit can be coupled to an energy storage device.